1. Field of the Invention
The present invention relates to a turbine engine, and particularly to an engine having a free turbine and provided with a compressor pre-swirl configuration.
2. Description of the Prior Art
It has been found that a conventional free turbine or multi-shaft gas turbine engine, when utilized with an electrical generator or utilized in similar auxiliary power systems, suffers from having a relatively slow response time when it is required to accelerate from minimum power conditions. In such a situation involving multi-shaft engines, the electrical generator is mechanically coupled to the power turbine rotor or rotors. Since the compressor shaft is independent of the power shaft, there is a lapse of time before the compressor will respond to develop the higher speed required from an engine minimum power condition to a maximum power condition. At low power requirements, although the power turbine shaft may rotate at a high relative speed, the compressor rotor speed, air mass flow and fuel flow is low. When a load is sensed by the engine, the compressor must accelerate to a higher speed by introducing more fuel to the combustor before a suitable air mass flow and gas horsepower can be produced to meet the new power requirements.
In a single shaft gas turbine configuration, the same shaft is mechanically coupled to the electrical generator and carries the power turbine rotors and the compressor rotors. Thus, the compressor responds rapidly to an increase in load since the power changes from minimum to maximum require only a change in fuel flow rate. A single shaft, however, has disadvantages, such as high starting torques and difficulties in matching compressor rotor speed to generator speed to provide optimum performance.
A free turbine engine is any gas turbine engine where the compressor shaft and the turbine power drive shaft are not mechanically connected or coupled. Free turbine gas turbine engines are frequently used in helicopter applications. Similarly, a sudden power demand is frequently made on the engine which, in the case of a landing, would be operated at low power requirements, that is, the helicopter rotor rotates at a constant velocity but with near-zero pitch. Just prior to touching down, the pitch of the rotor blades is increased in order to reduce the rate of descent to zero, thereby requiring an increase in the power output of the engine. However, due to the inherent delay in accelerating the compressor and thus the engine from minimum to maximum power, i.e., 3 to 5 seconds, the helicopter pilot must anticipate the fuel rate increase by the above time factor.
U.S. Pat. No. 4,222,703, issued to Schaum et al on Sept. 16, 1980 and assigned to Pratt & Whitney Aircraft of Canada Limited, describes an arrangement of a bleed valve adjacent the compressor communicating with hollow struts provided with a slot to form a jet flap preceding the compressor for the purpose of providing pre-swirl to the compressor by bleeding the compressor at an interstage location. This pre-swirl is utilized to improve stability of a free turbine engine at low power operation when the engine is designed for cruising performance levels.